Circuit for a rotary anode x-ray tube

ABSTRACT

A circuit is used for actuating the driving motor of a rotary anode of an X-ray tube. The driving motor is an a.c. asynchronous motor fed from a converter. The converter is provided with actuating means which increase the frequency of the feeding voltage during the starting of the rotary anode from an initial value to an end value so as to provide the shortest possible starting time. The device of the present invention is a component part of an X-ray apparatus feeding an X-ray tube.

United States Patent Seifert et al.

[ Aug. 27, 1974 I CIRCUIT FOR A ROTARY ANODE X-RAY TUBE [75] Inventors:Gerd Seifert; Kurt Franke, both of Erlangen, Germany [73] Assignee:Siemens Aktiengesellschaft, Elangen,

Germany 22 Filed: June12, 1973 21 Appl. No.: 369,394

Related US. Application Data [63] Continuation-impart of Ser. No.192,858, Oct. 27,

1971, abandoned.

8/1967 Wright, Jr 250/406 9/1969 Studtmann et al.... 3l8/231 X PrimaryExaminerWilliam F. Lindquist Attorney, Agent, or Firm-Richards & Geier;V. Alexander Scher [57] ABSTRACT A circuit is used for actuating thedriving motor of a rotary anode of an X-ray tube. The driving motor isan ac. asynchronous motor fed from a converter. The converter isprovided with actuating means which increase the frequency of thefeeding voltage during the starting of the rotary anode from an initialvalue to an end value so as to provide the shortest possible startingtime. The device of the present inventionis a component part of an X-rayapparatus feeding an X-ray tube.

[56] References Cited UNITED STATES PATENTS 4 Claims 7 Drawing Figures3,205,360 9/1965 Graves 250/406 l3 i2 if. control I oscmut I impulse.evice wider 6 fier onver ome tef PATENTED AUG 2 7 I974 sum 2 or 2CIRCUIT FOR A ROTARY ANODE X-RAY TUBE The present application is acontinuation-in-part of out copending patent application Ser. No.192,858 filed Oct. 27, 1971, now abandoned.

This invention relates to a circuit for actuating a driving motor,consisting of an asynchronous motor, of a rotary anode of an X-ray tube,with at least one converter which changes the frequency of the feedingvoltage relatively to the network frequency.

It is known to drive the rotary anode of an X-ray tube by-a one phaseasychronous motor with an auxiliary winding, the motor being fed from aconverter which supplies an outgoing voltage with a frequency of 150 Hzincreased relatively to the network frequency. When going over fromX-ray radioscopy to X-ray exposure the driving motor is switched on andthe rotary anode runs after starting with a speed of about 8,500 rpm.The X-ray tube is comparatively highly charged due to this high rotaryspeed of the anode required by frequency changing.

The starting time after the end of which the anode reaches its finalrotary speed when using a specific driving motor, depends in the knownconstruction upon the voltage applied to the driving motor. Since thisvoltage cannot be raised at will due to technical insulation reasons, areduction of the starting time is limited.

In order to move quickly from radioscopy to exposure it is desirable tokeep the starting time of the rotary anode as small as possible.However, in known construction with acceptable expenditure it is notpossible to lower the starting times below a lower end value of about0.8 seconds.

An object of the present invention is to improve the circuit of thedescribed type by making possible a very short starting time for therotary anode with a comparatively small technical circuit expenditure,particularly for a comparatively low feeding voltage of the drivingmotor.

Other objects will become apparent in the course of the followingspecification.

In the accomplishment of the objectives of the present invention it wasfound desirable to provide the converter with actuating means whichincrease the frequency of the feed voltage during the starting time ofthe rotary anode corresponding to the course of the pull-out torque froman initial value to a second higher value and then decreasing to an endvalue which is intermediate the initial and second value so as toproduce the shortest possible starting time. The present inventionproceeds from the consideration that the greater is the starting momentof an asynchronous motor, the lower is the frequency of the feedvoltage. The moment has its highest value during the starting period ineach time period when the speed frequency and the number of rotationsare so great that there is a pull-out torque. The present invention bychanging the frequency of the feed voltage, produces an increase in therotary moment of the driving motor during its starting, as compared tothe prior art device wherein the frequency of the feed voltage remainsconstant during the entire starting period, namely, it has its finalvalue. Thus the present invention provides a substantial shortening ofthe starting period.

The invention will appear more clearly from the following detaileddescription when taken in connection with the accompanying drawingshowing by way of example only, a preferred embodiment of the inventiveidea.

In the drawing:

FIG. I is a diagram showing the development of the rotary moment.

FIG. 2 is a block circuit of an embodiment of the present invention.

FIG. 3 is a diagram showing the construction of the converter.

FIG. 4 is a diagram illustrating the flow of voltages.

FIG. 5 is an example for the oscillator according to FIG. 2.

FIG. 6 is an example for the device 13 in FIG. 2.

FIG. 7 is the course of the voltage at the output of the device 13.

FIG. 1 illustrates the development of the rotary moment depending uponthe r.p.m. of an asynchronous motor for different frequencies of thefeed voltage.

Curve 1 is, for example, for a frequency of 50 Hz, curve 2 for afrequency of Hz and curve 3 for a frequency of about Hz. FIG. 1 showsclearly that the greater the initial rotary moment of an asynchronousmotor, the lower is the frequency of the feed voltage.

Therefore, in accordance with the present invention the driving motorfor the rotary anode of an X-ray tube, which is an asynchronous motor,is initially driven with a low frequency of, for example, 50 Hz and thefrequency is continuously raised during the starting corresponding tothe rise of the rotary speed of the driving motor until the desired endvalue is reached. In this manner the starting time can be greatlyreduced as compared to a device wherein the driving motor is fed with afeed voltage of end frequency already at the time it is switched on.

A best starting time period is attained when the frequency of the feedvoltage is increased corresponding to the development of the pull-outtorque above the end value to a value which provides the starting of theasynchronous motor up to the reaching of the final rotary speedcorresponding to the development of the pull-out torque, namely, with apull-out torque corresponding to the prevailing feed frequency. FIG. 1shows a curve 4 representing the rotary moment characteristic of thedriving motor for that particular case. The curve 4 has a startingfrequency of 50 Hz (curve I), a highest frequency of about 180 Hzcorresponding to the development of the pull-out torque (curve 3) and anend frequency of 150 Hz (curve 2), of the feed voltage. FIG. 1 indicateswith M1, M2 and M3 the pull out torques taking place at feed frequenciesof 50 Hz, 150 Hz and 180 Hz. Thus the pull-out torque develops dependingupon the feed frequency corresponding to the curve 4. Theabove-mentioned frequencies relate to a 50 Hz power system.

FIG. 2 illustrates a block circuit of an embodiment of the presentinvention. The driving motor for the rotary anode 26 of the X-ray tube27 is a one phase asynchronous motor having a rotor 5, a main winding 6and an auxiliary winding 7. The windings 6 and 7 are fed from converters8 and 9 which are connected to the outlet of a rectifier 10. Therectifier 10 can be connected by for the windings 6 and 7, can beadjusted. For that purpose serves a driving oscillator 12 the frequencyof which can be varied by a device 13 during the starting time period ofthe rotor 5 from an initial value to an end value. The oscillator 12drives the converters 8 and 9 through an impulse divider 14.

FIGS. 3 and 4 show the main construction of the converters 8 and 9 andthe development of voltages appearing at the windings 6 and 7. Theconverter 8 includes a pair of contacts and a pair of contacts 16 whichconnect the main winding 6 alternately to direct voltage. Similarly theconverter 9 has contact pairs 17 and 18 which connect the auxiliarywinding 7 alternately to direct voltage, namely, to the outlet voltageof the rectifier 10. The converters 8 and 9 are reverse pole switches sothat substantially rectangularly shaped voltages appear at the windings6 and 7.

In order to start the motor, the voltage at the main winding 6, thedevelopment of which is shown by the curve 19 of FIG. 4, is phaseshifted by 90 relatively to the voltage in the auxiliary winding 7, thedevelopment of which is shown by the curve 20 in FIG. 4. After theclosing of the switch 11, namely, when going over from illumination toexposure, as shown in FIG. 1, the contact pair 17 is closed at the timemoment 21 and the contact pair 18 opened and thereupon at the timemoment 22 the contact pair 15 is closed and the contact pair 16 isopened, at the time moment 23 the contact pair 17 is opened and thecontact pair 18 is closed, at the time moment 24 the contact pair 16 isclosed and the contact pair 15 is opened and at the time period 25 againthe contact pair 17 is closed and the contact pair 18 is opened, so thatthe described cycle is now periodically repeated.

The alternating actuation of contact pairs 15 to 18 is operated by theimpulse divider 14 which supplies the outgoing impulses of theoscillator 12 alternately to the converters 8 and 9. The impulse divider14 thus provides that the current flowing through the main winding 6 isshifted in phase relatively to the current flowing through the auxiliarywinding 7.

As indicated in FIG. 4, the frequency of the oscillator 12 must be fourtimes as great as the frequency of the feed voltage of the windings 6and 7. Starting with the assumption that to produce a quick start of therotor 5 the frequency of the feed voltage must be raised from 50 to 180Hz and then lowered to the end frequency of I50 HZ, then the frequencyof the outgoing voltage of the oscillator 12 must exceed the rangebetween 200 Hz and 720 Hz. The switch device 13 thus increasescontinuously the frequency of the oscillator 12 after the closing of theswitch 11 from 200 Hz to 720 Hz and then lowers it to 600 Hz. Thefunction followed by the frequency change and the time period duringwhich this takes place are fixed once and for all for a specific tube,namely, for a specific driving motor so as to produce the best possiblestarting time.

It was determined in actual practice, that the change of frequency ofthe feed voltage of the driving motor for the rotary anode, carried outin accordance with the present invention, makes it possible to shortenthe starting time period to less than 0.5 seconds.

In the actual construction of the device shown in FIG. 2, for thecontact pairs 15 to 18 are preferably used electronic switches, forexample, thyristors which operate substantially without inertia andlosses and with little wear.

Obviously, in accordance with the present invention I the one phaseasynchronous motor can be replaced by a three-phase asynchronous motor.In that case the two converters 8 and 9 are replaced by a converterproviding an outgoing three-phase voltage and operated by an actuatingdevice in such manner that its outgoing voltages passes through thedesired frequency range during the starting of the motor.

A braking of the driving motor and of the rotary anode can be providedin a simple manner by operating the converters so that in at least oneof the two converters a contact pair is closed during a few secondswhile the other one remains open, so that the corresponding motorwinding is subjected to direct voltage.

With reference to FIG. 1 there was described a change of the frequencyof the feed voltage corresponding to the development of the pull-outtorque (curve 4). This produces the best starting time.

FIG. 5 shows an example of the circuit connections of the operatingoscillator 12. The oscillator 12 includes a field effect transistor 28the operative stretch of which includes a condenser 29 and which isprovided with feed voltage through resistances 30 and 31. The transistor28 operates jointly with the condenser 29 as an oscillator. Theswingings of this oscillator are transmitted through a transistor 32serving as an amplifier which is connected with two resistances 33 and34 for coupling to the transistor 28 and the voltage source, to animpulse distributor 14 through the line 35. The oscillator 12 along withthe transistor 28 and the condenser 29 constitute a voltage-frequencyconverter. The outgoing frequency depends upon the voltage at thelocation 36 which is supplied through a coupling resistance 37. Thetimely running of this voltage is selected corresponding to the desiredcourse in time of the outgoing frequency of the oscillator 12. Thereforethe voltage at the location 36 initially rises according to FIG. 5 andthen drops to an end value which corresponds to the final r.p.m. of therotary anode.

FIG. 5 shows diagrammatically the course of the voltage in the line 35.It is apparent that the frequency starts by increasing with time.

As shown in FIG. 5, the actuating device 13 produces voltage at thelocation 36. It constitutes a function generator with a condenser 38,diodes 39 to 35, voltage dividers 46 to 52, a charging resistance 53 anda time member 54 (FIG. 6).

The outgoing voltage in the line 55 proceeds according to FIG. 7. Whenthe main switch 56 is closed, the diode 39 switches during the timeperiod t1 the poten' tial of the point 57 to the line 55. During thistime period the condenser 38 is being charged and the potential at thelocation 58 reaches after the expiration of the time period [1 thepotential at the location 57, so that now the diode 40 becomesconductive. After the expiration of the time period t1 the potential inthe line 55 now rises corresponding to the charging of the condenser 38through the resistance 53. During this rise the diodes 41 to 45 becomeconductive one after the other and switch the potential taken from thecorresponding voltage dividers upon the condenser 38. The charging ofthe condenser 38 takes place accordingly in stages as indicated in FIG.7. After the expiration of the time period [2 the time member 54 closesthe switch 59 and switches the potential taken from the voltage divider47 upon the condenser. The measurements of the voltage divider 47 aresuch that the voltage at the condenser 38 again drops somewhat, as shownin FIG. 7. I

By suitably dimensioning the parts of the function generator shown inFIG. 6, particularly the voltage dividers 46 to 52 and the chargingresistance 53, the desired run of voltage at the location 36 (FIG. 5)can be attained. The feed frequency of the motors Sto 7 proceeds thencorresponding to this voltage during the start of the rotary anode. Bysuitably dimensioning the function generator 13 it is thus possible tovary the feed frequency of the motors 5 to 7 during the starting of therotary anode and thus also the starting of the motor, so

that the smallest possible starting time is produced.

What is claimed is:

1. In combination with a feed voltage supplying a.c. source, an X-raytube having a rotary anode, an asynchronous a.c. motor driving saidanode, at least one frequency converter having an outlet connected withsaid motor and an inlet connected with said source for supplying themotor with an adjustable frequency feed voltage, and means connectedwith said converter for increasing the frequency of the feed voltageduring the starting of the anode corresponding to the rotary anodetorque from an initial frequency value to a second higher frequencyvalue and then decreasing to an end frequency value which isintermediate the initial and second value.

2. A device in accordance with claim 1, wherein said motor is a onephase motor with a main winding and an auxiliary winding, said devicehaving two converters, one of said converters being connected with saidmain winding and the other one of said converters being connected withsaid auxiliary winding, said means comprising a device shifting the feedvoltage for said auxiliary winding relatively to the feed voltage forthe main winding to the extent of 3. A device in accordance with claim2, wherein each frequency converter comprises a reverse pole switch,said converters having rectifier means connected with said source andsupplying direct voltage to said reverse pole switches, and a commonoscillator connected with said reverse pole switches and operating themwith variable frequency.

4. A device in accordance with claim 3, comprising an impulse dividerlocated between said oscillator and said converters and supplying theoutgoing impulses of said oscillator to each of said converters tochange its switching frequency.

1. In combination with a feed voltage supplying a.c. source, an X-raytube having a rotary anode, an asynchronous a.c. motor driving saidanode, at least one frequency converter having an outlet connected withsaid motor and an inlet connected with said source for supplying themotor with an adjustable frequency feed voltage, and means connectedwith said converter for increasing the frequency of the feed voltageduring the starting of the anode corresponding to the rotary anodetorque from an initial frequency value to a second higher freQuencyvalue and then decreasing to an end frequency value which isintermediate the initial and second value.
 2. A device in accordancewith claim 1, wherein said motor is a one phase motor with a mainwinding and an auxiliary winding, said device having two converters, oneof said converters being connected with said main winding and the otherone of said converters being connected with said auxiliary winding, saidmeans comprising a device shifting the feed voltage for said auxiliarywinding relatively to the feed voltage for the main winding to theextent of 90*.
 3. A device in accordance with claim 2, wherein eachfrequency converter comprises a reverse pole switch, said convertershaving rectifier means connected with said source and supplying directvoltage to said reverse pole switches, and a common oscillator connectedwith said reverse pole switches and operating them with variablefrequency.
 4. A device in accordance with claim 3, comprising an impulsedivider located between said oscillator and said converters andsupplying the outgoing impulses of said oscillator to each of saidconverters to change its switching frequency.